1. Technical Field
The invention relates to a method for the continuous manufacture of particle board and to an apparatus for carrying out this method. The invention has particular reference to a method for the continuous manufacture of chipboard, fiberboard or similar boards from a mat comprising material containing lignocellulose, and/or cellulose, and/or other electrically poorly conducting material in the form of particles, fibers or the like, together with at least one heat hardenable binder dispersed therein, said mat being brought into the board end-form by means of heat introduced at least in part by high frequency energy and pressure.
2. Prior Art
A method of the above named kind is known from German Auslegeschrift No. 27 22 348 (U.S. Pat. No. 4,221,950). In this known method the high frequency heating of the mat takes place between upper and lower plates of a capacitor arranged to introduce high frequency energy into the mat. The arrangement is however such that a small air gap of optimum size is always present between the electrode plates and the mat. This is achieved by regulating the separation of the electrode plates in dependence on the anode current of the electron tube used in the high frequency generator. The regulating circuit that is used features a coarse regulation which is used to vary the electrode plate separation until a predetermined desired value of the anode current is reached. A fine regulation is then superimposed on this coarse regulation to hold the anode current constant. The fine regulation is achieved not by varying the separation of the electrode plates but instead by varying an auxiliary capacitor connected in parallel to the working capacitor formed by the electrode plates. Should the variation in anode current be so great that it cannot be adequately dealt with by variation of the auxiliary capacitor then the coarse regulation once again comes into operation to adjust the separation of the electrode plates.
In order to ensure a flat surface of the mat within the working capacitor or, in the case of a precompressed mat, to prevent the mat expanding, and so as to always ensure a defined air gap, a run of an endless, recirculating, retaining band is tensioned against the surface of the mat passing through the working capacitor. In this manner it is possible to maintain a minimum spacing between the upper electrode plate and the surface of the mat, in particular in the range from 6 to 10 mm, and thereby to achieve good efficiency during high frequency heating. This arrangement has proved successful in practice.